A secondary battery for backup is preferably always kept in a state near the fully charged state in consideration of its application purpose. However, a secondary battery deteriorates due to long-term use, and the capacity of the secondary battery decreases. For this reason, it is necessary to replace the secondary battery whose capacity has become less than a predetermined capacity. It is therefore necessary to measure the capacity upon discharge of the secondary battery. However, the discharge of the secondary battery should be restricted to the necessary minimum. In a 10 kWh system for a cellular phone base station which uses a nickel metal hydride battery as a secondary battery for backup, two series of battery modules equipped with nickel metal hydride storage batteries are installed. The two-series battery modules are connected in parallel with a load and a battery charger. A conventional system uses a method of periodically measuring the capacity of each nickel metal hydride storage battery upon fully discharging the nickel metal hydride storage battery in one of the two-series battery modules (see reference: K. Saito, T. Shodai, A. Yamashita, and H. Wakaki, Proceedings of INTELEC '03 (The 25th International Telecommunications Energy Conference), p. 261, 2003). In this manner, the necessity of replacement of a nickel metal hydride storage battery is determined.
A nickel metal hydride storage battery is known to increase in internal resistance due to deterioration. That is, measuring the internal resistance of the nickel metal hydride storage battery can indirectly determine the necessity of replacement of the nickel metal hydride storage battery without discharging it. However, it is not easy to accurately measure the internal resistance of a nickel metal hydride storage battery. Furthermore, any concrete relationship between the internal resistance and the capacity of a nickel metal hydride storage battery was not known (see reference: Koji Nishio, Proceedings of '97 Battery Technology Symposium, p. 5-2-1, 1997).
Thereafter, the reference “A. Yamashita, H. Wakaki, K. Saito, and T Shodai, Proceedings of INTELEC '03 (The 25th International Telecommunications EnergyConference), p. 739, 2003” discloses that there is a relationship between a voltage drop amount after the end of charge and a capacity which has decreased due to deterioration. In addition, a method of correcting a voltage drop amount due to the temperature at the time of charge is proposed in “A. Yamashita, H. Wakaki, K. Saito, and T Shodai, Proceedings of INTELEC '05 (The 27th International Telecommunications Energy Conference), p. 291, 2005”.